Calculating Conductor Temp

[dithomas]

I have a 225A 277VAC 3 phase lighting panel begin feed by 1 4/0 CU 90C cable per phase. The total measured currect is 168 amps per phase. This is a main breaker panel with 90c lugs.

A resent infrared scan indicated an abolute temp on the cables of 70.1C. The scan photo shows a uniform temp on all three phases.

Two questions:
1. Does this temp at this current alarm anyone?
2. What is the formula to calculate the estimated conductor temp?

Thanks,
Dan

 

[wareagle]

Look at the rating of the main breaker. I am not aware of a breaker with 90C rating. Also look at table 310.16 in the NEC for conductor ratings. The rating at 75C and 90C are the temp that the conductor will reach in a 30C enviornment with a continous 3 hr load of rating shown. I don't have the NEC with me at this time.

 

[davidbeach]

As wareagle mentions, I doubt you have a breaker with 90^oC lugs. The 75^oC rating for 4/0 is 230A if the neutral isn't counted as current carrying. If you need to count the neutral as current carrying, you can derate from the 90^oC column and wind up at 208A, which can be protected by the 225A breaker.

70^oC seems rather high for conductors carrying only 73% of their rated ampacity. Are the connections all good, tight, and clean?

 

[wareagle]

70C may seem high but you need to take into account the enviornment and its temp. The rating in the NEC is for 30C or 86F. If it is hotter then you conductor will run hotter.

 

[subtech]

I agree that environment and ambient temp needs to taken into account when evaluating conductor temp.
However, because you stated that this is a lighting panel, I'd also suggest looking into the possibility of harmonic content in the current. If your load is primarily fluorescent lighting harmonic content may be high enough to contribute to your heating problem. Did you take your current measurements with a RMS type meter or an average reading meter?
Every situation is different, but I'd not neglect this possibility.

 

[dithomas]

Thanks for all the replies.

I will check the current wave form with our Fluke434.

Would the cable be subjected to the same K factor requirements as a transformer?

Dan

 

[resqcapt19]

The following is from SquareD.
quote:
--------------------------------------------------------------------------------
What are the allowable temperature rise limits for circuit breaker parts? Customer is doing thermal imaging.

Answer UL Surface Temperature Rise Limits
Non-metallic 60 deg. C
Metallic 35 deg. C
UL Terminal Temperature Rise
80% Rated CB 50 deg. C
100% Rated CB 60 deg. C
--------------------------------------------------------------------------------

Another question on the same site says that the rise is over a 40 deg. C ambient.
Don

 

[Marke]

As other have already suggested, the temperature rise could be due to an elevated temperature within the enclosure. I would use the IR scan to measure the temperature of non dissipating items within the enclosure to gain an indication of internal temperature.
The internal temperature will be a result of the total power dissipated within the enclosure and the ability of that enclosure to dissipate that energy. Harmonics may be a factor, but very often, I find that people do not consider the effect of power dissipated by cables, fuses, circuit breakers etc and commonly find enclosures with very elevated internal temperatures.
If the internal temperature is elevated above 45 degreesC, you will have an accelerated aging of the components within the enclosure and probably premature operation of protection devices.

Best regards,

Mark Empson

http://www.lmphotonics.com